Electron beam pumped vertical cavity surface emitting laser

ABSTRACT

A vertical external cavity surface emitting laser (VECSEL) structure includes a heterostructure and first and second reflectors. The heterostructure comprises an active region having one or more quantum well structures configured to emit radiation at a wavelength, λ lase , in response to pumping by an electron beam. One or more layers of the heterostructure may be doped. The active region is disposed between the first reflector and the second reflector and is spaced apart from the first reflector by an external cavity. An electron beam source is configured to generate the electron beam directed toward the active region. At least one electrical contact is electrically coupled to the heterostructure and is configured to provide a current path between the heterostructure and ground.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 13/523,681 filedJun. 14, 2012, now U.S. Pat. No. 9,112,332, which is incorporated hereinby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under U.S. ArmyCooperative Agreement No. W911NF-10-02-0008 awarded by the U.S. DefenseThreat Reduction Agency (DTRA). The Government has certain rights inthis invention.

SUMMARY

Embodiments discussed below include a vertical external cavity surfaceemitting laser (VECSEL) structure that includes a heterostructure andfirst and second reflectors. The heterostructure comprises an activeregion that includes one or more quantum well structures configured toemit radiation at a wavelength, λ_(lase), in response to pumping by anelectron beam. The heterostructure is disposed between the firstreflector and the second reflector and the heterostructure is spacedapart from the first reflector by an external cavity. An electron beamsource is configured to generate the electron beam, which is directedtoward the active region. At least one electrical contact iselectrically coupled to the heterostructure and is configured to providea current path between the heterostructure and ground.

According to various aspects, the VECSEL structure includes a substrate,which may be thinned to less than about 100 μm, the VECSEL structureincludes a heat sink, the heterostructure includes one or more layerswith one or both of n- and p-type doping in a range between about10¹⁷/cm³ to about 10²⁰/cm³, the electron beam comprises a stationaryelectron beam, and/or vias are disposed in at least one of theheterostructure and the second reflector.

Some embodiments involve methods of making a VECSEL structure. Somemethods include epitaxially growing a heterostructure comprising anactive region having one or more quantum wells structures configured toemit radiation having a wavelength, λ_(lase), in response to an electronbeam. The method includes forming a second reflector comprising adistributed Bragg reflector (DBR) and a contact. The heterostructure,second reflector, electrical contact, and a first reflector are arrangedso that the heterostructure is disposed between the first reflector andthe second reflector, the heterostructure is spaced apart from the firstreflector by an external cavity, and the contact makes electricalcontact with the heterostructure. An electron beam source is arranged togenerate the electron beam that pumps the active region.

According to some aspects, the heterostructure is epitaxially grown on afirst major surface of a substrate that is transparent to thewavelength, λ_(lase), the substrate is thinned to a thickness of lessthan 100 μm, and a dielectric DBR is deposited on a second major surfaceof the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an electron beam pumped verticalexternal cavity surface emitting semiconductor laser (VECSEL) inaccordance with some embodiments;

FIG. 2 illustrates a heterostructure used for a VECSEL in accordancewith some embodiments;

FIG. 3 shows an approach for the formation of a laser structureaccording to some embodiments involving epitaxially growing a reflectora series of alternating semiconductor layers on a substrate inaccordance with some embodiments;

FIG. 4 shows an approach for the formation of a laser structureaccording to some embodiments which involve a dielectric DBR depositedon the backside of an epitaxially grown heterostructure;

FIG. 5 shows an implementation in accordance with some embodimentswherein the laser component and e-beam source are located within thevacuum chamber and the external reflector and a majority of the externalcavity is outside of the vacuum chamber;

FIG. 6 shows an implementation in accordance with some embodimentswherein a laser structure includes a first reflector located outside thevacuum chamber and the heterostructure is epitaxially grown on asubstrate that is substantially transparent at the laser wavelength;

FIG. 7 illustrates a laser structure that includes vias which extendtoward and/or into the heterostructure in accordance with someembodiments;

FIG. 8 illustrates a heterostructure according to some embodiments, theheterostructure comprising an active region including quantum wellstructures arranged with increasing density closer to the side of theactive region where the e-beam enters the active region;

FIG. 9 shows an alignment of antinodes and quantum well structures inaccordance with some embodiments;

FIG. 10 illustrates a laser structure that includes a mesa feature inaccordance with some embodiments;

FIG. 11 depicts a laser structure that includes a second reflector whichis epitaxially grown on a substrate and an optional reflector disposedin a pattern on the heterostructure in accordance with some embodiments;

FIGS. 12 and 13 show laser structures wherein the heterostructure isgrown on a substrate that is substantially transparent at the laserwavelength; and

FIG. 14 depicts a base laser component of an example VECSEL structure inaccordance with some embodiments.

DESCRIPTION OF VARIOUS EMBODIMENTS

Semiconductor light emitting devices are used in a number ofapplications including data storage, displays, lighting, sensors, waterpurification, disinfection, UV curing, phototherapy, and medicaldiagnostics, among other uses. Light emitting devices that emit inspectral ranges suitable for these and other applications can befabricated based on a variety of material systems, including group III-Vand II-VI binary, ternary, and quaternary compounds and alloys andvarious combinations thereof. Vertical external cavity surface emittinglasers (VECSELs) are of interest due to the potential for high opticalpower coupled with the quality of spectral and spatial optical lasingcharacteristics. VECSELs have been used in optically pumped systems,however, the laser radiation emitted by the semiconductor gain region islimited by the pump wavelength. Furthermore, for wavelengths below 400nm, compact optical pump sources are not readily available. As describedbelow, semiconductor lasers pumped with an electron beam (e-beam) canachieve direct emission of radiation with VECSEL configurations thatprovide spectral and lasing characteristics and power output that issuitable for many more demanding applications.

Some embodiments discussed in this disclosure involve e-beam-pumpedVECSELs having an active region comprising distributed quantum wellstructures arranged to take advantage of resonant periodic gain andwhich are compositionally designed to emit at the desired laser emissionwavelength. The VECSEL implementations discussed herein can involvecontinuous-wave (cw), high power, compact, and lightweight e-beam pumpedlaser systems that provide high beam quality. The VECSEL structuresincorporate highly reflective distributed Bragg reflectors (DBRs), e.g.,¼ wavelength DBRs. The active region is disposed between the DBRs and atleast one of the reflectors is spaced apart from the active region toform an external cavity, which allows the generation of longitudinalsingle-mode operation, which can be useful for applications such asRaman Spectroscopy and/or other applications. In some implementations,the laser directly outputs UV radiation at wavelengths between 550 to200 nm, e.g. between about 400 nm to 250 nm, or between about 300 andabout 200 nm with spectral quality and output power suitable forspectroscopic applications. Depending on the material compositions usedfor the active region various wavelength ranges can be directly emitted.For example, InGaN is a useful material for the active region for blueor green visible light emission and AlGaN is useful to achieve UVemission.

FIG. 1 is a diagram illustrating an electron beam pumped semiconductorVECSEL 100 in accordance with some embodiments. The VECSEL 100 comprisesseveral main components including 1) a base laser component106—including a heterostructure 110 comprising an active region 101,contacts 120 configured to electrically discharge the heterostructure,and a reflector 160 (designated herein as the second reflector), 2) anexternal reflector 180 (designated herein as the first reflector) whichis spaced apart from the active region 101 to form an external cavity112, and 3) an e-beam pump source 102. The laser cavity 111 is disposedbetween the two reflectors 180, 160. The heterostructure 110 includes anactive region 101 which can comprise one or more quantum well structures121. Each quantum well structure 121 may include one quantum well ormultiple quantum wells separated from each other by relatively thinbarrier layers (thin relative to the spacers, for example). The VECSEL100 includes an electron beam source 102 configured to produce a beam ofelectrons (e-beam) 171 which are accelerated from the electron beamsource 102 and impinge on the heterostructure 110. The angle of thee-beam with respect to the surface of the base laser structure may rangefrom more than 0 degrees to less than 180 degrees. The electrons fromthe e-beam 171 disperse through the heterostructure 110 and/or otherlayers, forming an electron cloud 191.

The electron beam 171 can be created by an e-beam source operating atabout 1-50 kV acceleration voltage. The electrons from the source haveenergy that is much greater than the bandgap of the active regionmaterial and create electron hole pairs in the active region 101 of theheterostructure 110. Some of these electron hole pairs diffuse to thequantum wells, where they recombine to create the laser radiation. Theheterostructure 110 may include a number of semiconductor layers thatimprove the optical quality of the active region, such as layers thatprovide strain relief between epitaxially grown semiconductor layers ofdiffering composition.

The VECSEL 100 includes first 180 and second 160 reflectors that definethe laser optical cavity 111 including an external cavity 112 betweenthe first reflector 180 and the active region 101. The laser cavity 111can be configured so that reflections of laser light between the firstand second reflectors 180, 160 create a standing wave within the activeregion 101 such that the antinodes of the standing wave occur near thequantum well structures 121. This type of resonant periodic gainconfiguration in the active region can lead to optimal gain by thedevice.

In some configurations, the second reflector 160 includes layers ofmaterials arranged as a distributed Bragg reflector (DBR) and disposedproximate to heterostructure 110. For example, the material layers ofthe DBR may be selected so that the optical thickness (OT) of each layeris ¼ of the wavelength of the laser light to provide high reflectivityat the laser wavelength, λ_(lase). In some cases, to enhance dissipationof heat generated in the active region, the DBR 160 may be mounted on aheat sink (not shown).

The second reflector 180, which serves as an external out-couplingmirror, can also include a plurality of layers forming a DBR, e.g., a¼-wavelength DBR wherein each layer has an OT of ¼*λ_(lase). The layersmay be disposed on a concave support to form a concave reflector. Theconcave external mirror 180 serves to stabilize the laser emission andmay allow for single mode operation with one or more additional opticalelements within the cavity, even at higher power outputs, e.g., greaterthan about 10 mW. The external portion 112 of the laser cavity 111 can,in general, be any length that supports adequate gain in the activeregion and the desired optical characteristics, e.g., single modeoperation.

As shown in FIG. 1, the heterostructure, 110, first and secondreflectors, and the electron beam source may be disposed within a vacuumchamber 105.

The base laser component can be formed by various approaches, depending,for example, on the type of second reflector used. For example, oneapproach involves the use of an epitaxially grown second reflectorcomprising alternating layers of a semiconductor material. In thisscenario, the second reflector can be epitaxially grown on a substrateor a buffer layer on a substrate. The heterostructure including theactive region can then be deposited on the epitaxially grown DBR.

Some embodiments involve structural and/or material configurations fordischarging the electrons arising from the electron beam pumping of theheterostructure 110. FIG. 1 shows at least one contact disposed on orover the heterostructure 110. The contact 120 directly or indirectlymakes electrical contact with portions of the heterostructure 110 andprovides a current path for electrons to flow from the heterostructure110 to ground. Electrical contacts 120 can be formed of a metal, metalalloy and/or other conductive material. For example, suitable materialsfor the electrical contacts include one or more of Ti, Al, Au, V, Cr,Ni, Pd, Ag, and Pt. In some embodiments, the electrical contacts mayinclude a multilayer structure which includes one or more layers servingas adhesion layers and one or more layers serving as conductive layers,although this designation is somewhat arbitrary as both types of layersmay be electrically conductive. An adhesion layer may be deposited topromote adhesion between the heterostructure layer (or other layer) andsubsequent conductive layer. For example, suitable materials for anadhesion layer comprise Ti and suitable materials for a contact layercomprise Au and/or Al. In arrangements where the contact layer is nottransparent at the laser wavelength, an aperture 193 (also denoted anopening) in the contact layer 120 is provided at the location where thelaser radiation 192 emerges from the base laser structure. Inarrangements where the contact layer is optically transparent at thelaser wavelength, e.g., for indium tin oxide (ITO) or other transparentconductors, an aperture is not necessary.

To further facilitate current flow through the heterostructure 110, oneor more heterostructure layers may be doped with n-type or p-typedopants, increasing the electrical conductivity of the layers. Forexample, one or more heterostructure layers may be doped with an n-typeand/or p-type dopant at a level of about 10¹⁷ to 10¹⁹/cm³ to achieve aconductivity in the doped layers between about 0.01 (Ωcm)⁻¹ and 10⁴(Ωcm)⁻¹. The impurity concentration is typically less than 3% of thetotal atomic concentration in the material.

For example, the class of III-N semiconductors including GaN, AlN, InN,BN and alloys of these materials such as AlGaN, InGaN, InAlN, AlBN,GaBN, InBN, and InGaAlN may be employed to fabricate the devices. Thesematerials may be doped n-type by incorporating impurities such as Si,Ge, and Sn. These materials may be doped p-type by including impuritiessuch as Mg, Be, Zn, Cd, or C.

The class of III-As semiconductors including GaAs, AlAs, InAs and alloysof these materials including AlGaAs, InGaAs, and InAlAs may also beemployed. These materials may be doped n-type by incorporatingimpurities such as Si, Ge, Sn, S, Se, and Te. These materials may bedoped p-type by including impurities such as Mg, Be, Zn, Cd, Si or Ge.

The II-VI semiconductors, including for example oxides such as ZnO, MgO,CdO together with their alloys ZnMgO, CdMgO, CdMgO, may also be employedin the devices. These materials may be doped n-type by incorporatingimpurities such as Al, In, and Ga. These materials may be doped p-typeby incorporating N.

In some configurations, the heterostructure 110 includes a firstheterostructure region 126 and a second heterostructure region 127 withthe active region 101 disposed between the first heterostructure region126 and the second heterostructure region 127. The first heterostructureregion 126, the second heterostructure region 127, and the active region121, e.g., the one or more quantum well structures 125 and spacer layers124 may each have different doping. For example, in some cases the firstheterostructure region 126 and/or the second heterostructure region 127may have n-type doping. In some cases, the first heterostructure region126, the second heterostructure region 127, and one or more layers ofthe active region, e.g., the quantum wells, may have n-type doping. Insome cases, the first heterostructure region 126 and the secondheterostructure region 127 may not have additional doping, and thequantum wells may have p-type doping.

The heterostructure 110 can be formed by epitaxial growth on asubstrate, such as an AlN, GaN, sapphire, SiC, Si, GaAs or other type ofsubstrate, as depicted in FIG. 2. The type of substrate material useddepends in part on the type of material system in the active region. Theactive region 201 is epitaxially grown, e.g., by metal organic chemicalvapor deposition (MOCVD), on the substrate or a base layer 223 grown onthe substrate. For example, for active regions based on the AlGaNmaterial system, an AlN substrate may be used. For active regions basedon the InGaN material system, a GaN substrate may be used. Optionally,the substrate can be removed from the heterostructure 110, e.g., afterformation of other heterostructure layers as indicted by arrow 250.

The number and/or type of layers epitaxially grown as heterostructure110 will depend on the choice of substrate type. The heterostructure 110may include one or more nucleation layers 205, one or more defectreduction layers 206, and/or one or more strain relief layers 207. Theselayers can be grown on the substrate prior to, during and/or followinggrowth of the active region 201.

The active region 201 can include one or multiple quantum wellstructures 221, where each quantum well structure 221 includes at leastone quantum well layer 224. Six quantum well structures 221 are depictedin FIG. 2, wherein each quantum well structure 221 includes two quantumwells 224, although in general more than six or fewer than six quantumwell structures may be used and each quantum well structure 221 caninclude more or fewer quantum wells, e.g., 1-40 quantum wells. Thequantum wells 224 may comprise any direct bandgap material that producesthe desired radiation wavelength. In some cases a III-nitride material,e.g., InGaN, AlGaN, AlInN and AlGaInN, may be used. For example, in anAlGaN system, the quantum wells may comprise Al_(x)Ga_(1-x)N, where20≦x≦80. For InGaN quantum wells, the quantum wells may compriseIn_(y)Ga_(1-y)N, where 5≦y≦40. Those skilled in the art will appreciatethat the number of different material systems and compositions availableto achieve the disclosed embodiments is quite large and the examplesgiven in this disclosure provide a subset of the availablepossibilities.

As shown in FIG. 2, when multiple quantum wells 224 are present in thequantum well structures 221, thin barriers 225 separate the quantum welllayers 224. For AlGaN or InGaN material systems the thin barriers 225may comprise AlN, or may comprise GaN, for example. If two or morequantum well structures 221 are present in the active region 201, thequantum well structures 221 can be separated from one another by spacerlayers 222 which may be the same material as the thin barriers 225, butwith thickness greater than the thickness of the thin barriers.Additional layers 226 may be epitaxially grown below, above and/orwithin the active region 201.

The base laser component can be formed by various approaches, depending,for example, on the type of second reflector used. For example, oneapproach involves the use of an epitaxially grown second reflectorcomprising alternating layers of a semiconductor material. In thisscenario, the second reflector can be epitaxially grown on a substrateand the heterostructure including the active region can be epitaxiallygrown on the second reflector. FIG. 3 shows an approach for theformation of a base laser component that involves epitaxially growing aDBR (epi-DBR) 360 comprising a series of alternating semiconductorlayers on a substrate 365. For example, if AlN is used as for thesubstrate 365, then an epi-DBR comprising alternating layers of AlGaNand AlN could be grown directly on the AlN substrate. Using an epi-DBRhelps to promote heat dissipation from the active region because thesemiconductor layers of the epi-DBR can provide some heat sinking. Aheterostructure 370, including the active region is epitaxially grown onthe epi-DBR 360. After the epi-DBR+heterostructure subassembly 305 isformed, the substrate 365 may optionally be fully or partially removed,for example, using a laser liftoff process (LLO), mechanical polishingand/or dry/wet chemical etching. In some configurationsDBR+heterostructure subassembly 305 can be bonded epi-DBR side down to aheat sink 375 to enhance heat dissipation from the active region. Theheat sink may comprise a metal, metal-alloy or other materials havingsufficient thermal conductivity. For example, suitable materials for aheat sink include diamond, copper, copper-tungsten, aluminum, AlSiC,and/or other materials or material composites. Contacts 380 are disposedon or over the free surface of heterostructure 370.

Another approach for the formation of a laser structure, shown in FIG.4, involves the use of a dielectric DBR which is deposited on thebackside of an epitaxially grown heterostructure. Dielectric materialsused to form a dielectric DBR can have higher refractive index contrastthan semiconductor materials. Thus, for the same reflectivity, adielectric DBR can have fewer layer pairs when compared to the number ofsemiconductor layer pairs of an epitaxially grown DBR. However,dielectric materials have lower thermal conductivity which may be afactor for higher power devices that need heat dissipation from theactive region.

Formation of the base laser structure shown in FIG. 4 involvesepitaxially growing heterostructure 470 on a substrate 465. Thesubstrate 465 is fully or partially removed from the heterostructure470. After removal of the substrate, a dielectric DBR 460 is depositedon one surface of the heterostructure, e.g. the backside surface whichis the surface of initial epitaxial growth of the heterostructure 470.Contacts 490 are disposed on the other heterostructure surface. In theimplementation shown in FIG. 4, the DBR+heterostructure subassembly isarranged DBR side down on a heat sink 475 to provide for dissipation ofheat generated in the active region.

In some configurations, the external reflector may be positioned outsideof the vacuum chamber, as shown, for example, in FIGS. 5 and 6.Placement of the external reflector outside of the vacuum chamberfacilitates incorporation of additional optical elements within thelaser cavity, e.g., one or more of an etalon, a birefringent filter, afrequency converter such as a non-linear crystal and/or other opticalelements. FIG. 5 shows an implementation in which the base lasercomponent 506 and e-beam source 502 are located within the vacuumchamber 501 and the first (external) reflector and a majority of theexternal cavity is outside of the vacuum chamber 501. In theimplementation shown in FIG. 5, the electron beam 571 is incident on thesecond reflector 560 and forms an electron cloud at least in the secondreflector 560 and the heterostructure 521. In some cases, it may bebeneficial to use an additional reflector 565 disposed between theactive region of the heterostructure 521 and the first reflector 580.This additional reflector 565 may be compositionally similar to, butthinner than the 2nd reflector, e.g. the additional reflector may havefewer layers than the 2nd reflector. In some configurations, whereas thefirst and second reflectors 580, 560 are highly reflective (e.g.,greater than 99% reflective) to the laser radiation, the additionalreflector may be partially reflective at the wavelength of the laserradiation (e.g., about 40% to about 60% reflective). As previouslydiscussed, layers of the heterostructure 521 may be doped and/orgrounded electrical contacts 520 may be used to provide for dissipationof the electrons from the heterostructure layers. In some cases,contacts 520 disposed on the additional reflector 565 may not providesufficient conductivity between the heterostructure and ground. In thesecases, the contacts may be patterned directly on a surface of theheterostructure so that there is at least one opening in the contactlayer that provides access to the heterostructure surface. Theadditional reflector can be disposed directly on the heterostructurelayer within the contact layer opening. To enhance heat dissipation, thebase laser structure 506 may include one or more heat sink layers 575may be disposed at one or both sides of the heterostructure 521.

FIG. 6 shows another implementation for a laser structure that includesa first reflector 680 located outside the vacuum chamber 601. In thisimplementation, the heterostructure 610 which includes the active regionis epitaxially grown on a substrate 675 that is substantiallytransparent at the laser wavelength. Contacts 620 are arranged on thefree surface of the heterostructure 610, optionally with a heat sinkdisposed on the contacts 620. In some cases, as shown in FIG. 6, thecontacts 620 are patterned directly on the surface of theheterostructure 610, wherein the patterning includes at least oneopening that provides access to the heterostructure surface. The secondreflector layers 660 are disposed within the opening in the contactlayer 620. The electron beam source 602 in this example is locatedwithin the vacuum chamber 601 and generates an electron beam 671 whichis directed toward the patterned second reflector 660.

Arranging the contacts directly on the heterostructure can provide forsuperior discharging of the heterostructure layers when compared withcontacts disposed on an insulating component, such as a dielectric DBR.When both contacts and DBR need to be positioned on the sameheterostructure surface, patterning can be used so that the DBR andcontact is disposed on the heterostructure surface. This technique mayalso be useful when an epitaxial DBR is used in place of dielectric DBR,because the higher bandgap material within the epi-DBR can also lead toa decreased electrical conductivity between the heterostructure and thecontacts.

As illustrated in FIG. 6, the VECSEL structure allows additional opticalelements 650 to be inserted within the external cavity. Locating theexternal reflector 680 and external portion of the laser cavity outsideof the vacuum chamber facilitates incorporation of additional opticalelements within the external cavity. These additional optical elementscan be used to enhance spectral quality and/or provide for other opticalenhancements of the laser output. For example, the additional opticalelement 650 may comprise an etalon configured to enhance the spectralquality by reducing the lasing output modes to a few modes or a singlemode. In some implementations, the additional optical element 650 maycomprise a birefringent filter which is arranged to enhance spectralquality. In some implementations, the additional optical element 650 maycomprise a non-linear crystal that can be used to provide frequencymultiplication of the laser radiation. For example, a non-linear crystalmay be arranged to provide second harmonic generation (SHG) or higherfrequency multiples which would achieve radiation output at twice ormore of the frequency of the laser radiation. Using non-linear opticsallows for output wavelengths down to about 200 nm or less. Note thatthe various embodiments discussed herein are not limited to simpletwo-mirror cavity configurations. It can be beneficial for the frequencyconversion process to use a folded mirror configuration using three oreven more mirrors. The non-linear crystal can then be placed at aposition where the lasing mode waist is smallest and high photonconversion efficiency may be achieved.

Some applications, such as spectroscopy, require relatively high levelsof spectral quality in conjunction with relatively high output power. Toachieve the level of spectral quality useful for these types ofapplications, the e-beam source may be arranged so that the e-beam spotmade by the e-beam on the heterostructure is stationary. The e-beam mayhave a cross sectional diameter in a range of about 10 μm to about 500μm and/or a current between about 10 μA to about 1 mA, for example. Whenthe e-beam is not stationary, i.e., is scanned across the surface of theheterostructure, then even if the e-beam has a narrow width, e.g., about25 μm, multiple lasing modes will be produced as the e-beam is scannedacross multiple positions of the heterostructure. The multiple lasingmodes produced by the scanning operation result in reduced spectralquality when compared to a stationary beam, which is capable ofproducing few lasing modes or a single lasing mode. With the externalfeedback provided by a curved external mirror in a VECSEL configuration,the beam size of a stationary e-beam can be increased from a beam size(e.g., about 25 μm) typically used in a scanning implementation withlittle or no reduction in spectral quality. The larger e-beam spot sizeis also associated with an increase in optical power output. The VECSELlaser structures described herein can use a stationary e-beam having abeam diameter of about 200 μm and may achieve a line width smaller than0.5 nm at a power output of several tens of milliwatts and more. Forexample, the power density of the electron beam may be greater thanabout 20 kW/cm².

A stationary, higher power e-beam brings additional considerations withregard to heat dissipation because the stationary beam may causesignificant heating of the heterostructure, particularly at the locationof the beam spot on the heterostructure or other layers. As shown insome embodiments, heat can be dissipated using one or more heat sinksdisposed at one or both sides of the heterostructure. Additionally oralternatively, doping of one or more heterostructure layers increasesthe thermal conductivity of these layers, which allows these layers tomore efficiently conduct heat away from the active region.

To further enhance heat dissipation, the thermal connection between theheat sink and the heterostructure layers may be augmented using viasthat may be filled with metal or other electrically and/or thermallyconductive material, which protrude toward or into the layers of theheterostructure. The vias may be arranged to make a thermal connectionto the heat sink or contact that has a higher thermal conductivity thanthe layers surrounding the vias. An example of such a configuration isillustrated in FIG. 7. The laser structure of FIG. 7 includes aheterostructure 710 comprising an active region, which may be configuredto include one or more multiple quantum wells as previously discussed.The heterostructure 710 is disposed between a first (external) reflector780 and a second reflector 760. To enhance recycling of the radiationback to the active region, an additional reflector 765 can optionally bearranged between the heterostructure 710 and the external reflector 780.For example, in some cases, the additional reflector 765 may be thinnerand/or include fewer alternating pairs and/or have lower reflectivity tothe laser radiation in comparison with the second reflector 760.

As previously discussed, dielectric materials provide highly reflectiveDBRs, but generally have relatively low electrical and thermalconductivity, e.g. lower electrical and thermal conductivity thansemiconductor materials. Thus, the use of a dielectric DBR as the secondreflector may be an impediment to heat removal for the laser device. Asdepicted in FIG. 7, the low thermal conductivity of the dielectric DBRmay be mitigated while still retaining the highly reflective propertiesby using a dielectric DBR 760 a in conjunction with an epitaxially grownsemiconductor DBR 760 b. The dielectric DBR 760 a and the epi-DBR 760 bare used together as the second reflector 760. The higher thermalconductivity of the semiconductor layers of the epi-DBR 760 b increasesthe overall thermal conductivity of the second reflector 760 incomparison to the thermal conductivity of a second reflector that reliessolely or predominantly on dielectric materials. In variousimplementations, the second reflector 760 and/or the additionalreflector 765 may be formed using only semiconductor layers or onlydielectric layers, however, second reflectors comprising a combinationof semiconductor and dielectric layers takes advantage of the higherthermal conductivity of the semiconductor materials as well as thehigher refractive index contrast of the dielectric materials. Asillustrated in FIG. 7, if a heat sink 775 and/or contact layer 720 isused at a surface of the heterostructure 710 through which the laserradiation 791 emerges, then an aperture 792 may be provided in thecontact 720 and/or heat sink 775 layers to facilitate extraction of thelaser radiation 791.

As illustrated in FIG. 7, to promote conduction of electrical and/orthermal energy through the layers of the laser structure, a number ofvias 750 may be etched or otherwise formed to extend from the heat sink775 and/or contact 720 at least partially through the reflectors 760,765 and/or the heterostructure 710. The vias 750 can be filled with ametal or other good electrical and thermal conductor, such as Au or Ag.For example, each of the vias 750 may have a cross sectional area ofabout 100 μm to provide a selected level of augmentation in the thermaland/or electrical conductivity between the heat sink 775 and/or contact720 and the heterostructure 710.

It can be helpful to position the quantum wells within the active regionto achieve a homogeneous carrier distribution in all quantum wells. Insome implementations, the density of quantum wells within the activeregion may increase as a function of distance starting from the side orsurface of the active region farthest from the location where theelectron beam first enters the active region. This configuration resultsin a higher density of quantum wells in the portion of the active regionwhere the e-beam is stronger and a larger percentage of the carriers arecreated by the electron beam.

FIGS. 8 and 9 depict some portions of the laser structure of FIG. 7 withthe active region 701 shown more detail, where like reference numbersrefer to similar structures in FIGS. 7-9. FIG. 8 illustrates aheterostructure 710 comprising an active region 701 including sixquantum well structures 721 arranged with increasing density closer tothe side of the active region 701 where the e-beam enters the activeregion 701. In this particular implementation, each quantum wellstructure 721 comprises three quantum wells 725 with thin spacer layers724 between the quantum wells 725. Thick spacer layers 726 are arrangedbetween the quantum well structures 721.

FIG. 9 shows the active region 721 of the device illustrated in FIG. 7.The active region 721 includes a portion 901 having a relatively higherdensity of quantum well structures; a second portion 902 having amoderate density of quantum well structures; and a third portion 903having a lower density of quantum well structures. A depiction of thelaser radiation 940 having a wavelength, λ_(lase), is superimposed onthe active region 701. The quantum well structures 721 of the activeregion 701 are spaced apart by the thick spacers 726-1, 726-2, 726-3.The thickness of the spacer layers 726-1, 726-2, 726-3 in regions 901,902, 903 varies so that the antinodes 950 of the laser radiation 940 arealigned with quantum well structures 721, and the density of the quantumwell structures 721 varies with distance from the point where the e-beamenters the active region. The thickness of a quantum well structure,QW_(t), plus the distance of a spacer layer, SLm_(t), is a multiple ofhalf the wavelength within the material of the primary longitudinal modeof laser radiation, λ_(lase), where m=1, 2, and 3 in portions 1, 2, and3, respectively. As depicted in FIG. 9, in the first portion 901,QW_(t)+SL1 _(t)=1×(λ_(lase)/2n); in the second portion 902, QW_(t)+SL2_(t)=2×(λ_(lase)/2n); and in the third portion 903, QW_(t)+SL3_(t)=3×(λ_(lase)/2n), where n is the effective refractive index of thematerial.

There are other ways to achieve varying density of the quantum wellswith distance while maintaining a resonant periodic gain structure. Forexample in some configurations, the quantum wells structures closer tothe electron beam source may include more quantum wells when comparedthe quantum well structures farther from the quantum well source.

A laser cavity may have multiple longitudinal modes and multiple lateralmodes. Some laser implementations may include one or more features, suchas mesas, that provide enhanced lateral mode confinement of the laseroutput radiation. FIG. 10 illustrates a laser structure 1000 thatincludes a mesa feature 1011 having a circular cross-section which canbe formed by partially etching away the semiconductor layers of theheterostructure 1010. For example, the cross sectional diameter of thecircular mesa feature 1011 may be in a range of about 10 μm to about 500μm. Although a circular mesa feature is described here as an exampleshape, it will be appreciated that one mesas having various othercross-sectional shapes, e.g., ellipse, square, rectangular, etc., couldbe used to achieve lateral mode confinement. The heterostructure 1010 isdisposed between the first 1080 and second reflectors 1060. FIG. 10shows a possible orientation for the electron beam source 1002, thefirst and second reflectors 1080, 1060, and the heterostructure 1010,although various other orientations are possible.

A patterned dielectric DBR 1065 is deposited on the mesa feature 1011shown in FIG. 10. A higher refractive index would result below thenon-etched areas of the mesa features 1011 and the lateral mode of thelaser radiation would be better confined in these regions 1011. Metalcontacts 1020 can be formed around the mesa features 1011 on the surfaceof the heterostructure 1010.

FIGS. 11 through 15 provide several examples for laser structures. Theseexamples provide only a subset of the various arrangements that could beused for e-beam pumped VECSELS, which are considered to fall within thescope of this disclosure. For example, each of the examples illustratedin FIGS. 11 through 15 could be used in conjunction with variousconfigurations discussed herein, such as contacts disposed at one orboth sides of the heterostructure, heat sinks disposed at one or bothsides of the heterostructure, vias disposed at one or both sides of theheterostructure, patterned dielectric and/or epitaxially grownsemiconductor DBRs and/or patterned contacts on one or both sides of theheterostructure and/or lateral confinement features, including mesafeatures used with patterned DBR and/or contact layers.

FIG. 11 depicts a laser structure that includes a second reflector 1160which is epitaxially grown on a substrate 1175. The substrate 1175 maybe opaque or transparent at the wavelength of the laser radiation. Alaser heterostructure 1110 including an active region is epitaxiallygrown on the second reflector 1160. An optional additional reflector1165 is epitaxially grown in a pattern on the heterostructure 1110. Theadditional reflector 1165 may be electrically isolating and may comprisean epitaxially grown DBR or a dielectric DBR. One or more contacts 1120are disposed on the heterostructure surface. In another variation, thecontacts 1120 may be patterned on the heterostructure surface, e.g.,with a number of openings in the contact layer 1120. The additionalreflector 1165 may be disposed on the heterostructure 1110 within theopenings in the patterned contact layer 1120. A heat sink layer (notshown in FIG. 11) may be disposed at either side of the heterostructure.

FIG. 12 shows another alternative laser structure. In thisconfiguration, the heterostructure 1210 is grown on a substrate 1275that is substantially transparent at the laser wavelength. An additionalreflector 1275, e.g., with patterning as discussed in connection withFIG. 11, may optionally be disposed on the heterostructure 1210. Theadditional reflector 1265 may comprise semiconductor layers epitaxiallygrown on the heterostructure and/or may comprise deposited dielectriclayers and/or may be electrically isolating. After growth of theheterostructure 1210 and/or additional reflector 1265, the substrate1275 is thinned, e.g., to a thickness of about 10 μm to 200 μm. Thesecond reflector 1260, comprising a dielectric DBR, is deposited on thefree surface of the thinned substrate 1275. A heat sink layer (not shownin FIG. 12) may be disposed on either side of the heterostructure 1210.As an example, the transparent substrate may comprise AlN or sapphire.Sapphire may be implemented as a low cost alternative.

FIG. 13 shows an alternative laser structure that includes aheterostructure 1310 grown on a substrate 1375 that is substantiallytransparent at the laser wavelength. An additional epitaxially grownreflector 1365 may be disposed between the transparent substrate 1375and the heterostructure 1310. The first reflector 1380 is spaced apartfrom the heterostructure 1310 by an external cavity. The secondreflector 1360 may be epitaxially grown (epi-DBR) or deposited(dielectric DBR) on the free surface of the heterostructure 1310.Contacts 1320 are disposed on the transparent substrate 1375. One ormore heat sink layers 1370, 1372 may be disposed over the contacts 1320and/or may be disposed at either side of the heterostructure 1310. Inthis configuration, the laser light 1391 emerges through the transparentsubstrate 1375 and is directed toward the external reflector 1380.

FIG. 14 is a more detailed view of a base laser component of an exampleVECSEL structure. The first (external) reflector and the external cavityare not shown in FIG. 14. The device layers are formed from left toright starting with the GaN substrate. An optional additional reflectormay comprise a DBR, which is partially reflective at the laserwavelength, is grown on the GaN substrate. The optional additionalreflector may comprise 10 periods of Al_(0.2)Ga_(0.8)N/GaN, where theAl_(0.2)Ga_(0.8)N layers are about 48.8 nm thick and the GaN layers areabout 46.8 nm thick having a reflectivity of about 40% to about 60% atthe laser wavelength. For example, the thickness of the AlGaN layer maybe about λ_(laser)/4n_(AlGaN)(λ_(lase)) and the thickness of the GaNlayer can be about λ_(lase)/4n_(GaN)(λ_(lase)).

The active region includes multiple active region elements, such asabout 10 periods of active region elements, is grown on the optionalreflector. Each of the active region elements includes a double quantumwell structure based on InGaN. Each active region element may include inorder the following layers: an InGaN pre-strain layer(In_(0.03)Ga_(0.97)N, 35.3 nm thick), a first thin spacer (GaN, 5 nmthick), a first quantum well, (In_(0.18)Ga_(0.82)N, 3 nm thick), asecond thin spacer (GaN, 5 nm thick), a second quantum well,(In_(0.18)Ga_(0.82)N, 3 nm thick), a thick spacer (GaN, 21.7 nm thick),and an AlGaN carrier confinement and strain management layer(Al_(0.2)Ga_(0.8)N, 20 nm thick).

The second reflector may comprise one or more of epitaxial semiconductorlayers and non-epitaxial dielectric layers arranged as a DBR. Forexample, in come configurations, the second reflector may comprise anepitaxial DBR comprising GaN/AlGaN, such as 10.5 periods ofGaN/Al_(0.2)Ga_(0.8)N, where the GaN layers are about 46.8 nm thick andthe AlGaN layers are about 48.8 nm thick. A second reflector portioncomprising a non-epitaxial dielectric DBR may be deposited on theepitaxial DBR. For example, the dielectric DBR may comprise 4 periods ofSiO₂/TiO₂, wherein the SiO2 layers are 78.8 nm thick and the TiO₂ layersare 52.3 nm thick. If two DBR mirrors are joined together to create asingle hybrid DBR the highest reflectivity at the target wavelength isobtained when the high refractive index material of the first DBR is incontact with the low refractive index material of the second DBR. Thusthe hybrid DBR will have contact between GaN and SiO₂ because GaN has ahigher refractive index than AlGaN and SiO₂ has a lower refractive indexthan TiO₂. The dual DBR that includes an epitaxial semiconductor portionand a non-epitaxial dielectric can be used to achieve a specifiedthermal conductivity and reflectivity. The epitaxial semiconductorportion can have higher thermal conductivity than the non-epitaxialdielectric portion, whereas the non-epitaxial dielectric portionprovides higher reflectivity than the epitaxial-semiconductor portion.In some cases, both the epitaxial DBR and the non-epitaxial DBR are¼-wavelength DBRs.

A number of values and ranges are provided in various aspects of theimplementations described. These values and ranges are to be treated asexamples only, and are not intended to limit the scope of the claims.For example, embodiments described in this disclosure can be practicedthroughout the disclosed numerical ranges. In addition, a number ofmaterials are identified as suitable for various facets of theimplementations. These materials are to be treated as exemplary, and arenot intended to limit the scope of the claims.

The foregoing description of various embodiments has been presented forthe purposes of illustration and description and not limitation. Theembodiments disclosed are not intended to be exhaustive or to limit thepossible implementations to the embodiments disclosed. Manymodifications and variations are possible in radiation of the aboveteaching.

The invention claimed is:
 1. A vertical external cavity surface emittinglaser (VECSEL) structure, comprising: a first reflector; a secondreflector; a heterostructure comprising an active region that includesone or more quantum well structures comprising AlGaInN configured toemit radiation at a wavelength, λ_(lase), having a range from about 200to about 400 nm, in response to pumping by an electron beam, the activeregion disposed between the first reflector and the second reflector andspaced apart from the first reflector by an external cavity; an electronbeam source configured to generate the electron beam which is directedtoward the active region; and one or more electrical contacts, each ofthe contacts having the same electrical potential and electricallycoupled to the heterostructure and configured to provide a current paththat discharges electrons arising from the electron beam pumping of theheterostructure.
 2. The structure of claim 1, further comprising avacuum chamber, wherein the second reflector, the heterostructure, theat least one electrical contact are disposed within the vacuum chamberand the first reflector is disposed outside the vacuum chamber.
 3. Thestructure of claim 1, wherein the heterostructure comprises a M-Nsemiconductor having one or more layers with n- or p-type doping in arange between about 10¹⁷/cm′ to about 10²⁰/cm³ and the n-type dopingincludes one or more of Si, Ge, and Sn and the p-type doping includesone or more of Mg, Be, Zn, Cd, and C.
 4. The structure of claim 1,wherein the electron beam source is configured to produce a stationaryelectron beam.
 5. The structure of claim 1, wherein the electron beamhas at least one of: a cross sectional diameter of about 10 μm to about500 μm; and a power density of the electron beam is greater than about20 kW/cm².
 6. The structure of claim 1, further comprising an opticalelement disposed within the optical cavity.
 7. The structure of claim 1,wherein each of the quantum well structures are located substantially atan antinode of the radiation.
 8. The structure of claim 1, furthercomprising a substrate comprising AlN or GaN or sapphire.
 9. Thestructure of claim 8, wherein the substrate has a thickness less thanabout 100 μm.
 10. The structure of claim 1, further comprising a heatsink arranged to remove heat from the active region.
 11. The structureof claim 10, wherein the heat sink is adjacent the second reflector. 12.The structure of claim 10, wherein the heat sink comprises one or moreof diamond, copper, copper-tungsten, aluminum, and AlSiC.
 13. Thestructure of claim 1, wherein the first and second reflectors havereflectivity greater than 99% at the wavelength, λ_(lase).
 14. Thestructure of claim 1, wherein the structure is configured to provideoutput radiation having a line width smaller than 0.5 nm at a poweroutput of more than 20 mW.
 15. A vertical external cavity surfaceemitting laser (VECSEL) structure, comprising: a first reflector; asecond reflector; a heterostructure comprising an active region thatincludes one or more quantum well structures comprising AlGaInNconfigured to emit radiation at a wavelength, λ_(lase), having a rangefrom about 200 to about 400 nm, in response to pumping by an electronbeam, the active region disposed between the first reflector and thesecond reflector and spaced apart from the first reflector by anexternal cavity, the active region having a resonant periodic gainstructure such that antinodes of a standing wave in the active regionoccur near the quantum well structures; an electron beam sourceconfigured to generate the electron beam which is directed toward theactive region; and one or more electrical contacts electrically coupledto the heterostructure and configured to provide a current path thatdischarges electrons arising from the electron beam pumping of theheterostructure, each of the electrical contacts having the sameelectrical potential.
 16. A method, comprising: epitaxially growing aheterostructure comprising an active region having one or more quantumwell structures comprising AlGaInN and configured to emit radiation inresponse to an electron beam, the radiation having a wavelength,λ_(lase), in a range of 200 to 400 nm; forming a second reflectorcomprising a distributed Bragg reflector (DBR); forming one or moreelectrical contacts electrically coupled to the heterostructure, each ofthe electrical contacts having the same electrical potential; arrangingthe heterostructure, the second reflector, the electrical contact, and afirst reflector so that the heterostructure is disposed between thefirst reflector and the second reflector and the heterostructure isspaced apart from the first reflector by an external cavity; andarranging an electron beam source to generate the electron beam thatpumps the active region, wherein the electrical contact is configured toelectrically discharge electrons arising from electron beam pumping ofthe heterostructure.
 17. The method of claim 16, wherein: epitaxiallygrowing the heterostructure comprises epitaxially growing theheterostructure on a first major surface of a substrate; and thinningthe substrate to a thickness less than about 100 μm.
 18. The method ofclaim 17, wherein the substrate comprises AlN or GaN.
 19. The method ofclaim 17, wherein forming the second reflector comprises depositing adielectric distributed Bragg reflector (DBR) on a second major surfaceof the thinned substrate.
 20. The method of claim 16, wherein:epitaxially growing the heterostructure comprises epitaxially growingthe heterostructure on a first major surface of a substrate; andremoving the substrate.
 21. The method of claim 20, wherein forming thesecond reflector comprises depositing a dielectric distributed Braggreflector (DBR) on the heterostructure.